The influence of magmatic differentiation on the oxidation state of Fe in a basaltic arc magma

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Subduction zone basalts are more oxidized than basalts from other tectonic settings (e.g., higher Fe 3+/∑Fe), and this contrast may play a central role in the unique geochemical processes that generate arc and continental crust. The processes generating oxidized arc magmas, however, are poorly constrained, although they appear inherently linked to subduction. Near-surface differentiation processes unique to arc settings might drive oxidation of magmas that originate in equilibrium with a relatively reduced mantle source. Alternatively, arc magmas could record the oxidation conditions of a relatively oxidized mantle source. Here, we present new measurements of olivine-hosted melt inclusions from a single eruption of Agrigan volcano, Marianas, in order to test the influence of differentiation processes vs. source conditions on the Fe 3+/∑Fe ratio, a proxy for system oxygen fugacity (fO 2). We determined Fe 3+/∑Fe ratios in glass inclusions using μ-XANES and couple these data with major elements, dissolved volatiles, and trace elements. After correcting for post-entrapment crystallization, Fe 3+/∑Fe ratios in the Agrigan melt inclusions (0.219 to 0.282), and their modeled fO 2s (δQFM +1.0 to +1.8), are uniformly more oxidized than MORB, and preserve a portion of the evolution of this magma from 5.7 to 3.2wt.% MgO. Fractionation of olivine±clinopyroxene±plagioclase should increase Fe 3+/∑Fe as MgO decreases in the melt, but the data show Fe 3+/∑Fe ratios decreasing as MgO decreases below 5wt.% MgO. The major element trajectories, taken in combination with this strong reduction trend, are inconsistent with crystallization of common ferromagnesian phases found in the bulk Agrigan sample, including magnetite. Rather, decreasing Fe 3+/∑Fe ratios correlate with decreasing S concentrations, suggesting that electronic exchanges associated with SO 2 degassing may dominate Fe 3+/∑Fe variations in the melt during differentiation. In the case of this magma, the dominant effect of differentiation on magmatic fO 2 is reduction rather than oxidation. Tracing back Agrigan melts with MgO>5wt.% (i.e., minimally degassed for S) along a modeled olivine fractionation trend to primary melts in equilibrium with Fo 90 olivine reveals melts in equilibrium with the mantle beneath Agrigan at fO 2s of δQFM +1 to +1.6, significantly more oxidized than current constraints for the mantle beneath mid-ocean ridges. © 2012 Elsevier B.V..

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Earth and Planetary Science Letters